Victorian Oval Brooch: Possibly Blue John

Cathy bought this brooch at an antique mall. The tag said "Victorian Agate Brooch", but we're not so sure about that. We think it may be Blue John rather than Agate. It's 2½ inches in length, and appears to be gold-plated white metal.

The outer edges of the stone have agate-like stripes, but the center part looks to us more like Blue John (https://en.wikipedia.org/wiki/Blue_John_(mineral)). Cathy and I are both non-experts, though.

There are no identifying marks anywhere on the brooch, but some wear on the back edge reveals a little of what looks like silver. Therefore we believe it is gold-plated white metal,

We tried to date the brooch based on the hinge and catch designs. We found a web site on the topic: https://www.realorrepro.com/article/Dating-brooch-fasteners.

It has a tube hinge. It's a bit hard to tell, but we think the tubes are attached individually, rather than having a base pad. According to the site linked above, that makes it no earlier than 1850 and no later than 1910.

The catch looks to have been replaced at some point after 1930. It's a modern safety catch, but it's on a base plate, which is soldered to the brooch, The pin is short, similar to modern brooches, but in the hinge photo above it looks like the pin may have been cut down at the hinge end and re-attached.

Therefore we believe this is a Victorian brooch, made between 1850 and 1910.

There is a mounting loop for the included safety chain, which could possibly have been added when the catch was replaced.